drm/amdgpu: read reg in each iterator of psp_wait_for loop

v2: fix the SOS loading failure for PSP v3.1

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Acked-by: Huang Rui <ray.huang@amd.com> (v1)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>